One molding method for fiber-reinforced composite materials is a method using a prepreg obtained by impregnating a matrix resin, composed mainly of a thermosetting resin, into reinforcing fibers, and it is employed in a wide range of uses from sports leisure applications to aircraft applications. Molding of a fiber-reinforced composite material using an intermediate base material comprising the aforementioned prepreg is carried out by laminating the prepreg, and then subjecting it to heating or to heating and pressing to cure the thermosetting resin as the matrix resin.
Prepregs can be classified into low temperature-curing (80-100° C.), moderate temperature-curing (110-150° C.) and high temperature-curing (160-200° C.) types, depending on the curing temperature.
Low temperature-curing prepregs can be cured and molded near 90° C., while having a very wide range of selection of subsidiary materials to be used in molding and allowing use of resin dies for molding, thus permitting lower equipment investment. They are therefore advantageous for molding highly variable parts in small amounts. However, since the curing and molding are carried out near 90° C., it is not possible to obtain fiber-reinforced composite materials with excellent heat resistance.
Moderate temperature-curing prepregs serve primarily for molding of generic products for sports and leisure purposes, and allow curing and molding at temperatures near 130° C., and while they produce fiber-reinforced composite materials with excellent mechanical properties, it is not possible to obtain superior heat resistance.
High temperature-curing prepregs are subjected to curing and molding at temperatures near 180° C., and are used, for example, mainly for molding of fiber-reinforced composite materials to be supplied in fields that require excellent heat resistance, such as aircraft production, providing molded articles with extremely excellent heat resistance.
In order to obtain fiber-reinforced composite materials with high heat resistance, curing is accomplished at high temperature, thereby producing both high mechanical properties and high heat resistance. However, two major problems are associated with high temperature-curing systems.
One problem is resin flow. For fiber-reinforced composite material prepregs, the resin viscosity is usually set in consideration of manageability at room temperature. Because of the large difference between room temperature and the curing temperature in a high temperature-curing system, the viscosity of the impregnated resin tends to be reduced and resin flow readily occurs. This results in sections of resin deficiency or excess in the fiber-reinforced composite material. Variations in the matrix resin distribution not only affect the thickness and outer appearance of the molded article, but are also associated with reduced mechanical properties, and cracking. This is of particular concern for heat-resistant materials, which may suffer adverse effects to their long-term heat resistance.
Another problem is the heat resistance of mold materials and subsidiary materials. A fiber-reinforced composite material is usually molded using a molding die. Examples of molding methods include “hand lay-up” methods in which repetition of a procedure wherein a resin is impregnated into a reinforcing fiber material such as a cloth while applying it along a molding die, or wherein a prepreg having the resin already impregnated in a reinforcing fiber material is applied along a molding die, is followed by curing, and then removal from the die to obtain a molded article; resin transfer molding methods in which a reinforcing fiber material such as a cloth is set in a molding die, and then a resin is injected therein and cured and then removed from the die to obtain a molded article; molding compound methods in which a molding material comprising a mixture of a resin with a reinforcing fiber material cut into staple fibers is injected into a molding die and then cured, and then removed from the die to obtain a molded article; and methods in which rectangular columnar molding dies (mandrels) are used as inserts to hold and cure a prepreg in an I-beam or H-beam shape, and are then removed from the die to obtain a molded article.
The molding dies used in such molding methods are made of various types of materials including metal, resin, wooden and gypsum. Metal molding dies have excellent heat resistance and durability, but are expensive due to the effort and labor required for their formation, while their high specific gravity renders them problematically heavy. Resin molding dies and wooden molding dies, on the other hand, have inferior heat resistance and durability. At the current time, gypsum molding dies are in wide use because of their satisfactory balance between performance and cost.
In molding methods using such gypsum molding dies, the molding is preferably carried out at no higher than 130° C. because of the heat resistance of the molding dies themselves or the subsidiary materials, and this generally hampers molding of highly heat-resistant materials using high temperature-curing prepregs that require molding temperatures of 180° C. or higher. When it is attempted to cure and mold such high temperature-curing prepregs at low temperature or moderate temperature, molding is usually impossible to accomplish, or else an exceedingly long time is necessary and the molded article has vastly reduced heat resistance.
A widely employed solution is to carry out primary curing of the high temperature-curing prepreg at low temperature or moderate temperature, with addition of a curing agent and curing accelerator to the resin composition, and to subsequently carry out secondary curing at high temperature, but it is generally impossible to avoid reduction in mechanical properties and heat resistance, and even when mechanical properties are maintained, the heat resistance has been inevitably reduced due to addition of the curing agent and curing accelerator.